This invention relates to an amplifier arrangement comprising a driver stage, which the driver stage comprises an input signal terminal, a differential pair having a first and a second input terminal, a common terminal, and a first and a second output terminal. The second input terminal is coupled to a first supply terminal by means of a unidirectional element. An output stage, of the amplifier comprises an output signal terminal, a first output transistor of a first conductivity type having a base coupled to the first input terminal, an emitter coupled to the first supply terminal, and a collector coupled to the output signal terminal, and a second output transistor of a second conductivity type having a base coupled to the second output terminal, an emitter coupled to a second supply terminal, and a collector coupled to the output signal terminal. The first output transistor, the differential pair and the unidirectional element constitute a translinear network.
Such an amplifier arrangement is suitable for general purposes and can be used advantageously in integrated semiconductor circuits, the driver stage being particularly suitable for realizing class AB operation of the output transistors.
Such an amplifier arrangement is known from the article entitled "Low voltage operational amplifier with rail-to-rail input and output ranges", which article has been published in the "IEEE Journal of Solid-State Circuits" of December 1985, Vol. SC-20, No. 6, pp. 1144-1150. FIG. 4 of this article shows an amplifier stage which forms part of the operational amplifier described and which, in order to obtain class AB operation of the output transistors, comprises a further unidirectional element coupled between the second input terminal and the first supply terminal, in series with the unidirectional element, and a current source coupled between the second input terminal and the second supply terminal. In operation, the current source consequently supplies both unidirectional elements with a current, resulting in a reference voltage between the second input terminal and the first supply terminal, the driver stage realising the class AB operation on the basis of this reference voltage.
A drawback of the known amplifier arrangement is the supply voltage which it requires, which voltage, as a result of the presence of the unidirectional elements and the current source should be at least equal to two junction voltages plus one saturation voltage. As a result of this, the required supply voltage is larger than, for example, a voltage supplied by a standard button cell (1.2 V), which prohibits the use of the known amplifier arrangement in some circuits requiring a comparatively low supply voltage, such as, for example, hearing-aid circuits.